From cf4534c9daf2ead4ce530d374f0a06a95bd98693 Mon Sep 17 00:00:00 2001
From: Kun Qin <kuqin@microsoft.com>
Date: Wed, 23 Apr 2025 10:39:32 -0700
Subject: [PATCH] OvmfPkg: SmmControl2Dxe: Avoid double initialization

This change adds a build time PCD to prevent the SmmControl2Dxe driver
from re-initialization the MM related registers.

This register is to be used when MmControlPei is already executed in PEI
phase and completed setting up the MM control registers.

Signed-off-by: Kun Qin <kun.qin@microsoft.com>
---
 OvmfPkg/OvmfPkg.dec                       |  4 ++++
 OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.c   | 16 +++++++++-------
 OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf |  1 +
 3 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/OvmfPkg/OvmfPkg.dec b/OvmfPkg/OvmfPkg.dec
index 2078cf27a8..edc72a98fe 100644
--- a/OvmfPkg/OvmfPkg.dec
+++ b/OvmfPkg/OvmfPkg.dec
@@ -485,3 +485,7 @@
 
   ## This feature flag indicates the firmware build needs the qemu variable service.
   gUefiOvmfPkgTokenSpaceGuid.PcdQemuVarsRequire|FALSE|BOOLEAN|0x77
+
+  ## Informs modules whether the platform firmware supports Standalone MM.
+  #
+  gUefiOvmfPkgTokenSpaceGuid.PcdStandaloneMmEnable|FALSE|BOOLEAN|0x100065
diff --git a/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.c b/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.c
index e7438b18dd..5632b84af9 100644
--- a/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.c
+++ b/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.c
@@ -201,13 +201,15 @@ SmmControl2DxeEntryPoint (
   // this bit is clear after each reset.
   //
   SmiEnableVal = IoRead32 (mSmiEnable);
-  if ((SmiEnableVal & ICH9_SMI_EN_APMC_EN) != 0) {
-    DEBUG ((
-      DEBUG_ERROR,
-      "%a: this Q35 implementation lacks SMI\n",
-      __func__
-      ));
-    goto FatalError;
+  if (!FeaturePcdGet (PcdStandaloneMmEnable)) {
+    if ((SmiEnableVal & ICH9_SMI_EN_APMC_EN) != 0) {
+      DEBUG ((
+        DEBUG_ERROR,
+        "%a: this Q35 implementation lacks SMI\n",
+        __func__
+        ));
+      goto FatalError;
+    }
   }
 
   //
diff --git a/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf b/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
index d20198cab7..c31aee411c 100644
--- a/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
+++ b/OvmfPkg/SmmControl2Dxe/SmmControl2Dxe.inf
@@ -69,6 +69,7 @@
 
 [FeaturePcd]
   gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
+  gUefiOvmfPkgTokenSpaceGuid.PcdStandaloneMmEnable
 
 [Depex]
   TRUE